Electro-optical character display system



| FISCHLE ETAL 3,067,413

ELECTRO-OPTICAL CHARACTER DISPLAY SYSTEM Filed April 27, 1960 Dec. 4, 1962 2 Sheets-Sheet l FIG. 1

INVENTORS LISELOTTE FISCHLE WERNER KULCKE PAUL SCHWEITZER JURGEN TRAPP fin 7 AGE/V7 Dec. 4, 1962 L. FISCHLE ETAL ELECTRO-OPTICAL CHARACTER DISPLAY SYSTEM Filed April 27, 1960 2 Sheets-Sheet 2 27:: ZTQNF g a g a ...T J T J 358 United States Patent Ofifice 3,067,413 Patented Dec. 4, 1962 3,067,413 ELECTRO=OPTICAL CHARACTER DISPLAY SYSTEM Liselntte Fischle, Sindeifingen, Werner Kulcke, Boblingen, Paul Schweitzer, Sindeifingen, and Jurgen Trapp, Boblingen, Germany, assignors to International Business Machines Qorporation, New York, N.Y., a corporation of New York Filed Apr. 27, 1960, Ser. No. 25,080 Ciaims priority, appiication Germany May 9, 1959 5 Claims. (Cl. 340-334) The present invention relates to character display systems and particularly to improved high-speed character display systems employing arrays of electro-optically ac tive light controlling devices.

In data processing machines, the problem of the optical display of intermediate and final results is of outstanding importance. Even more important is the problem of the output devices which re ister the results and, if necessary, print explanatory passages and should be adapted to the operating speed of the computing systems, which is mostly very high. In order to meet the great number of different requirements to be fulfilled by display and output, the said problems have so far been realized in separate arrangements.

For the display of information which is mostly present in the form of electric signals, it has so far been customary to use glow lamp fields which, however, have the disadvantage of being very diflicult to survey. In order to avoid this disadvantage of difiicult observation, it has already been proposed to use mechanically movable digit carriers as display means. However, such systems have not proved useful for the purposes of data processing machines because of the great number of structural elements to be mechanically moved and the susceptibility to trouble caused thereby as well as their inertia. Moreover, it has been proposed to use display devices consisting of a great number of selectively operable incandescent or glow lamps which through slots or masks project specially designed or normal characters to a ground-glass screen. However, in view of their long adjusting times and their relative low light output, such arrangements are not expected to succeed in practice. An additional disadvantage of these arrangements is their high susceptibility to trouble due to the extremely large number of individually selectable incandescent or glow lamps used therein, the complicated control logic and the relatively great space requirements. A further approach to the problem, which has been used in the past, is a specially designed cathode ray tube having a phosphorescent screen, wherein a series of successive deflection means, electronic lenses and masks serve to display the characters to be represented on the screen of said tube. Although that arrangement has a very high display speed and a good light output, it is so expensive that it is uneconomic for most applications.

The output arrangements used up to now have mostly been mechanical printing units which, on the one hand, could not be used as display means and, on the other hand, did not operate satisfactorily in many applications in view of their high technical requirements and their relatively low printing speed.

An output unit having a very high operating speed consists of the above mentioned cathode ray tube in connection with a xerographic printing device. However, because of its extremely high cost and its relatively great susceptibility to trouble, this arrangement is uneconomic for most applications.

In order to avoid these disadvantages, the present invention has for an object the provision of an arrangement for the optical display of printed characters based on information having the form of electric pulses, wherein on one or a plurality of electro-optically active crystals penetrated by a ray beam and arranged between crossconnected polarizers there are arranged, at opposite surfaces thereof penetrated by said radiation, electrodes and counter-electrodes which due to their selective connection with an electric voltage source influence the ray beam penetrating them in such a manner that the projection thereof, depending on the electrodes selected, represents a predetermined character. Particular advantages of this arrangement are the very short adjusting time, the very high light output due to the use of an external light source, and the possibility of displaying a great number of alphanumerical characters with a low number of controllable element with good contrasts. Moreover, such systems are extremely small and cheap to manufacture due to the low number of required structural elements and the possibility of using printed circuits.

Another object of the invention is to provide an arrangement of the type described which utilizes a minimum number of character forming elements.

A further object of the invention is to provide an arrangement of the type described in which mutual interference between the character forming elements is reduced.

Still another object of the invention is to provide an arrangement of the type described in which the quality of the surface finish of the crystals need not be high.

A further object of the invention is to provide an arrangement of the type described which is not affected by humidity or other advance atmospheric conditions.

A further object of the invention is to provide an improved electro-optical character display system.

In a particularly simple embodiment of the invention, the individual electrodes are arranged in the form of line elements by which it is possible to compose all of the characters to be displayed. It has proved particularly useful that the entity of all electrically selectable electrodes provided for each display position is mounted in one single plane perpendicular to the direction of irradiation on one or a plurality of electro-optically active crystals. Accordingly, it is possible to arrange several crystals in one plane side by side and to mount the electrodes required for displaying a character on this crystal surface composed in the manner of a mosaic. It may, however, also be advisable to arranged the electrically selectable electrodes provided for each display position in a plurality of planes lying one behind the other perpendicular to the direction of irradiation and on a plurality of electro-optically active crystals or groups of crystals in such a manner that a mutual electric influence of overlapping or very closely spaced electrodes is avoided. It is obvious that to each of these planes provided with selectively controllable, line-shaped electrodes there is also associated a transparent counter-electrode which is preferably areal. According to another embodiment of the invention, the counter-electrodes are formed of a plurality of selectively electrically controllable, entirely or partly transparent, electrically conductive elementary areas which allows a partial activation of the line-shaped electrodes arranged on the opposite crystal surface in such a manner that the number of electrodes required for representing all characters is reduced.

Thus it is possible to subidivide the counter-electrode, which consists of a transparent area and comprises a whole character field, into two rectangular segments, so that e.g. a circular electrode arranged on the other crystal surface becomes effective as a semicircular electrode. It has proved particularly useful to make the counter-electrodes, too, in the shape of lines and to make them assume the shape of the electrodes arranged on the crystal surface facing them. In that arrangement, the line of the counterelectrode may either be uninterrupted or again subdivided into individual selectively controllable elements. In a particularly advantageous embodiment of the invention, the line-shaped counter-electrodes mounted on a crystal surface perpendicular to the direction of irradiation are arranged in such a manner that they coincide With the projection of the line-shaped electrodes arranged on the op posite crystal surface. It is, however, also possible to arrange the line-shaped counter-electrodes mounted on a crystal surface perpendicular to the direction of irradiation in staggered relationship to the projection of the lineshaped electrodes mounted on the opposite crystal surface so that the electro-optically modified crystal regions form an acute angle with the rays penetrating the crystal, so that the lighted area approximately corresponds to the amount of staggering.

It has also proved to be of particular advantage that the crystals provided with electrodes are individually or in groups brought into an immersion bath having a high dielectric strength. This method has the advantage that the electrodes which are individually and separately controlled may be arranged in a very closely spaced relationship without the danger of an electric breakdown to noncontrolled electrodes on the application of high electric voltages. A further advantage obtained by using an immersion bath consists in the fact that it is possible to select an immersion fluid having such an optical index of refraction that any unevenness of the crystal surface remains without effect, so that crystals having not finely worked surfaces may be used. That eliminates the necessity of polishing the crystals or taking care of the quality of their surfaces in any other manner. In order to reach this object, it is necessary that the index of refraction of the immersion fluid correspond about to the mean value of the refractive indices of the immersed crystal.

An additional advantage may be reached with immersion, i.e. it is possible in this manner to dissipate the heat produced in the crystal. This heat dissipation may be further improved by moving the immersion fluid with respect to the crystal, which may be achieved e.g. by flowing or forced rotating of this fluid. If it is desired in the arrangement of the invention to use electromagnetic radiations of a given frequency range, a fine-granular sub stance of the nature of a Christiansen filter may be added to the immersion fluid. A further advantage of the immersion bath resides in the fact that it is also possible to use hygroscopic or air-sensitive crystals.

It is well known that for achieving a high contrast of the characters to be represented the occurrence of divergent radiation should be avoided as far as possible. For this purpose, honeycomb diaphragms for eliminating any stray light causing lightening of the background are provided at one or a plurality of positions in the path of radiation.

In order to make full use of the divergent radiation emanating from a point-shaped light source, it is also possible to provide birefringent compensation crystals the thickness and refractive index difference of which are so selected that the phase difference occurring in the controllable crystals, when in their uncontrolled condition and in the presence of divergent rays due to birefringence, is canceled.

A particularly simple embodiment of the arrangement according to the invention is obtained by providing a number of crystals arranged successively in the radiation path corresponding to the number of characters provided for each display position, which crystals, besides the counterelectrodes, include line-shaped electrodes corresponding to the shape of the individual characters. In this arrangement, each crystal carries a controllable electrode representing a given character. If a given electrode is connected to a voltage source, the image of the respective character will appear on the ground-glass screen. The electrodes are so designed that in their unselected condition they have no noticeable influence on the path of radiation.

In order to reduce the number of electrodes required for representing a given number of characters, a special design of character representation is employed. For that purpose, there are provided on one 'or a plurality of cry-s tals a number of individually controllable electrodes having the form of straight and bent lines which is smaller than the number of characters to be represented. By a suitably controlled selection of certain electrode combinations, the corresponding characters are rendered visible on the ground-glass screen. It has proved to be of particular advantage to arrange on the controllable crystals eifecting the display of the characters line-shaped electrodes in the shape of two superimposed circles, an upright cross bisecting said circles and a concentric diagonal cross as well as a rectangle surrounding the aforementioned electrodes.

The path of radiation influenced by the above described means is rendered visible, e.g., through a Fresnel lens (annular lens), through a ground-glass screen, through fluorescent layers or by the illumination of photosensitive substances. By using photosensitive substances or by the additional use of Xerographic methods, respectively, it is possible to employ the arrangement of the present invention successfully as a high-speed printer.

The electro-optically active crystals employed in the above described arrangement may preferably comprise compounds of the type of the primary alkaline phosphates and the isomers thereof, for example NH I-I PO KH PO KD PO the alkaline tartrates and the isomers thereof, for example Rochelle salt C H O NaK or the barium titanates and the isomers thereof, for example BaTiO The compensation crystals to be used in accordance with the invention preferably consist of compounds of the types NH4NO3, K2S206, IiHCzO Na3PO4, (NH4)ZC4H4OG, or rutile.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a diagrammatic representation of an electrooptical shutter, illustrating the basic phenomena utilized in the invention.

FIG. 2 illustrates one embodiment of the arrangement according to the invention, comprising a plurality of electrodes mounted on a corresponding plurality of separate crystals.

FIG. 3 shows an arrangement designed in accordance with the invention for displaying specially designed characters by means of twenty-seven selectively controllable electrodes arranged on two crystals succeeding each other within the path of radiation.

FIGS. 4 and 5 are a schematic representation of the electrodes used in the arrangement of FIG. 3.

FIG. 6 graphically represents certain of the characters which can be displayed by means of the arrangement shown in FIGS. 3, 4 and 5.

FIG. 7 is a diagrammatic fragmentary view of another embodiment of the invention in which the crystals are immersed in a suitable immersion bath.

Referring to FIG. 1, there is shown a basic arrangement for utilizing the electro-optical effect, which in the pertinent technical literature is referred to as an electrooptical shutter. In this figure, an electro-optically active crystal 5 is arranged between a polarizer 4 and an analyzer 8. The forward directions of the polarizer 4 and of the analyzer 8 form an angle of as indicated by the double arrows. In front of analyzer 8, there is arranged a ground-glass screen 9. The light coming from a bright, point-source of light 1 is set parallel by a collimater lens 2 and directed through the arrangement as a parallel ray beam 3. In the embodiment of FIG. 1,

the crystal 5 is an optically uniaxial ammonium-dihydrogen-phosphate crystal cut perpendicularly to its optical axis. Consequently, the polarization state of the light passing through the crystal in the direction of its optical axis is not changed in the field-free crystal. Therefore, normally no light will reach the ground-glass screen 9 through the crossed polarizers 4 and 8. The irradiated surfaces 5a and 5b of the crystal 5 are covered in a wellltnown manner with semitransparent electrodes 6 and 7, one of which is grounded, whereas the other one may be connected with an electric voltage source through a switch 11. Due to the electro-optical effect, the crystal is thereby rendered optically anisotropic in the direction of irradiation and changes the polarization state of the light beam originally linearly polarized in the forward direction of the polarizer 4. Depending on the voltage applied to the crystal, it is possible to direct more or less light through the analyzer 8, so that the ground-glass screen 9 is lighted as a function of the applied voltage. The maximum brightness, which theoretically corresponds to one half of the light directed into the arrangement, is obtained when the voltage applied across the crystal 5 equals the M2 voltage characteristic for the crystal. As is known, the M2 voltage is the voltage in the presence of which the path lengths of the two radiation components in the direction of the axes of the index ellipsoid of the crystal on passing through the crystal have a path length difference of one half wave length.

Exemplary A/ 2 voltages are:

Ammonium-dihydrogen-phosphate 8.75 kv., Potassium-dihydrogen-phosphate 7.1 kv.,

and for Potassium dihydrogen phosphate substituted with heavy water 2.5 kv.

As substantial brightening effects occur already at lesser path length differences than one half wave length, 02-fold values of the above indicated \/2 values may also be sufficient in certain circumstances.

The arrangement shown in FIG. 1 is based on the assumption that the crystal 5 in its voltage-free condition is isotropic in the direction of irradiation. It is, however, also possible to use crystals which are birefringent in the direction of irradiation already in the voltage-free condition. In that case, a second crystal may be placed in front of or behind the electro-optically active crystal for cancelling the original birefringency.

The effect described in the above example is composed of two partial effects: The direct action of the electric field on the electron configuration of the crystal building blocks (direct electro-optical effect) and the effect seeondarily produced by the piezoelectric deformation (elasto-optical effect). Of these, the former is independent of frequency in the range of radio waves, whereas the latter will disappear above the mechanical resonant frequency of the crystal, which for a crystal having a size of approximately 1 cm. is in the order of 100 kilocycles per second. Of the entire electro-optical effect, approximately 70% are due to the direct, 30% due to the elasto-optical effect. If only a very small part of the crystal surface is covered with electrodes and the predominant part is free of electrodes, the electrode-free regions will obstruct the piezoelectrical deformations and thus cause the elasto-optical effect to disappear. If in this case the frequency of the applied voltage is not just equal to one of the mechanical inherent frequencies of the crystal, brightening will occur in an arrangement according to FIG. 1 only immediately between the electrodes and within a narrow margin of approximately 0.5 mm. about the electrodes. According to the present invention, this effect is utilized for displaying alphanumerical characters. Of course, the arrangement illustrated in FIG. 1 may also be designed with a plurality of successive crystals without the individual crystals disturbing each other.

Referring to FIG. 2, there is shown an arrangement wherein the radiation emanating from a point-source of light 1 passes through a condenser lens 32. Beyond the lens 32 there is arranged an apertured diaphragm 33 beyond which an additional lens 2 is provided for collinearizing the light. Beyond this lens 2 there is arranged a polarizer 4 which passes only light having the direction of polarization shown by the double arrow. The linearly polarized light leaving the polarizer 4 passes through ten crystals 56 to 59 (only five of which are shown in FIG. 2 in order to simplify the drawing) which besides a transparent counter-electrode 60 to 69 covering an entire crystal surface, each have one of ten different electrodes 70 to 79 having the shape of the digtis l, 2, 3 9 and 0. Beyond the crystal '59 there is arranged an analyzer 8 the forward direction of which forms an angle of 90 with the forward direction of the polarizer 4. The crystals 50 to 59 consist of compounds cut perpendicularly to the optical axis and optically isotropic in that direction, so that the polarization state of the light is not changed in these crystals in the field-free condition thereof. In this manner, no light will pass through the analyzer 8 to the ground-glass screen 9 arranged beyond the latter. The electrodes 70 to 79 of the crystals 50 to 59 are provided with conductors 80 to 89 by means of which they may be selectively connected through the high voltage power supply unit 90 to electric voltages. The semitransparent counter-electrodes 60 to 69 provided on the opposite crystal surfaces are grounded through the conductor 92. The voltage supply unit 90 is controllable by means of a device 93 to which the information is supplied via the conductors 94 in the form of electric pulses. If now a voltage is applied through the voltage supply unit 90 to one of the electrodes 70 to 79, the crystal region between the areal counter-electrode and the electrode to which a voltage is applied becomes optically anisotropic, so that the polarization state of the light passing through this crystal region is changed. The light the polarization state of which has been changed in this manner can now pass through the analyzer 8, so that the figure corresponding to the electrode to which a voltage is supplied is rendered visible on the ground-glass screen 9. Due to the fact that the electrodes representing the individual characters consist of very thin conductors and hardly obstruct the beam passing through the crystal, the brightness of the digits projected to the ground-glass screen is very great. Moreover, the figure appearing on the ground-glass screen is not disturbed by the line-shaped electrodes arranged on the other crystals. In order to avoid stray light disturbing the contrast of the image, the honeycomb diaphragm 95 is arranged between the analyzer 8 and the ground-glass screen 9.

Referring to FIG. 3, there is shown an arrangement for displaying specially designed representations of characters, wherein the number of controllable electrodes is substantially lower than the number of characters to be displayed. The light from source 1 is projected through the condenser lens 32 to the apertured diaphragm 33. The image of this apertured diaphragm is converted by the collimator lens 2 into a parallel path of radiation 3 passing through the polarizer 4, the crystal 96, the crystal 97, the analyzer 8 and the honeycomb diaphragm 95. The crystals 96 and 97 are provided with individually controllable electrodes to 127 and 101 to 119 which, depending on their selective control, influence the light beam in such a manner that one of the characters represented in FIG. 6 will appear on the ground-glass screen 9. The electrodes 12% to 127 arranged on the crystal 96 correspond to the electrode arrangement shown in FIG. 5 and are connected through eight separate conductors 178 to 177 to the voltage supply unit 90. On the opposite surface of the crystal 96 there is provided the same arrangement of electrodes (not visible in FIG. 3) with the exception that those electrodes are commonly connected and grounded via one single conductor 180. The electrodes 101 to 119 arranged on the crystal 97 correspond 9%. If a given character is supplied to the control unit 93 in the form of electric pulses, the voltage supply unit a ii is o erated in such a manner that it supplies a voltage to all of those electrode elements 101 to 127 which are required for the optical representation of that character. if e.g. the digit 8 isapplied in the form of electric pulses through the seven conductors 94 to the control unit 93, the voltage supply unit 98 is set up so that it supplies a voltage to the conductors leading to the electrodes 120 to 127. As the counter-electrode is connected to ground potential, an electric field is produced between the electrodes and counter-electrodes which influences the crystal regions included therein in such a manner that they become optically anisotropic and which changes the polarization state of the linearly polarized radiation passing through the polarizer 4. This radiation, which when passing through the uninfluenced crystal regions has such a position with respect to the forward direction of the analyzer that no light whatever can pass, is so changed in the regions which have become anisotropic that it can pass through the analyzer 8 and causes the image of the electrodes 12-6 to 1.217 shown in FIG. to become visible on the groundglass screen 9 as a light image of the digit In this connection it should be noted that the electrodes provided on the two crystals are so thin and in the ncnselected condition disturb the path of radiation to such a small extent that they do not become visible on the groundglass screen 9. For the representation of other characters, electrodes of the crystals 96 and/ or the crystal 97 are simultaneously connected to the high voltage source, so that the resulting light image represents a predetermined character. Thus, e.g., the digit 9 is represented by applying a voltage to the electrodes 120 to 123, M d, 125 and 126. In the same manner, the digit 3 is represented by supplying the electrodes 101, 102, 113, 124, i.- and 126 with voltage, and the digit 6 is represented by supplying the electrodes 120, 123, 109, 110 and 12 to 127 with voltage. In a similar manner, the letter A is represented by supplying the electrodes 108, 199, 116), 12s, 1123, 1 3d, 1%, 114 and 118 with voltage, the letter B is represented by supplying the electrodes 108 to 111, 101, 12%, 121, 113, 124, 125 and 167 with voltage, the letter C is represented by supplying the electrodes 1&9, 11 12%, 125 and 12.6 with voltage, etc. Certain of the characters are shown in H6. 6, and it will be apparent from these examples that well-formed numerals, letters, and special symbols can. be readily formed by energizaticn of selected electrodes.

FIG. 7 illustrates one manner in which the electrooptical active crystals such as 96, 97, may be immersed in a suitable bath. The remainder of the apparatus is not shown, in order to simplify the drawing, but can be arranged as shown in FIG. 6.

Crystals 96 and 97 are suspended, by supporting means not shown, in a container 199, at least two walls 191 and 193 of which are transparent to the radiant energy. Suitable feed-throughs or bushings permit the wires from the electrodes counter-electrodes to pass out of the container. The container is filled, at least sufficiently to fully immerse the crystals, with a liquid of high dielectric strength, such as a suitable oil, having optical properties ich permit passage of .he radiant energy with the least .e loss.

The use of a fluid of high dielectric strength lessens the chance of breakdown occurring between electrodes at different potentials, therefore permitting closer spacing of the electrodes, which decreases the over-all size of the apparatus and provides better resolution of the displayed images.

By selection of a fluid having an index of refraction which is comparable with that of the crystals, a higher degree of surface roughness of the crystals can be tolerated. Additionally, by dispersion of suitable granular material therein, a Christiansen filter effect can be obtained to render the apparatus more effective in a given portion of the spectrum. Immersion of the crystals also removes any adverse effects thereon due to atmospheric conditions.

A further advantage of immersion of the crystals is that the heat energy generated therein is readily dissipated by the bath. Natural convection can be used, or as shown in FIG. '7, a forced circulation may be obtained by use of a circulatory pump 195, driven by a suitable motor 197, and connected to the tank or container by conduits 198 and 199.

It is, of course, possible in the instance where multiple displays are provided, to utilize a common bath for all of the crystals, since there is no interaction. Also, it is apparent that in the case of multiple displays, many of the components may be common to more than one set of crystals, for example, the light source, collimating lenses, polarizers, analyzers, honeycomb filter and display screen.

From the foregoing, it is apparent that the present invention provides an improved electro-optical display system which utilizes a minimum number of character forming elements, which by location on a plurality of successively disposed optically active crystals, reduces the mutual interference therebetween. Also provision of such an arrangement in an immersion bath of high dielectric strength permits closer spacing of electrodes/ with resultant decrease in size and increase in resolution, reduced tolerance of crystal surface finish, and better heat dissipation.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An electro-optical character display system comprising a plurality of electro-optically active crystals disposed between crossed polarizers and traversed by a beam of nadiation, a plurality of character segment forming electrodes disposed on one face of each of said crystals, a counter electrode disposed on the other face of each of said crystals, means for selectively connecting said electrodes and counter electrodes across a source of potential whereby selected portions of said beam are influenced during passage through said crystal, means for receiving said selected portions of said beam, and immersion bath means for immersing said crystals in an immersion bath fluid having a high dielectric strength.

2. An electro-optical character display system as claimed in claim 1, in which the immersion bath fluid has an index of refraction substantially the same as the mean index of refraction of said crystals.

3. An electro-optioal character display system as claimed in claim 1, in which the immersion fluid is circulated in said immersion bath to remove heat produced in said crystals.

4. An electro-optical character display system. as claimed in claim 1, in which the immersion fluid contains fine granular substance to form a Christiansen filter, whereby only radiation of a predetermined spectral region is permitted to pass.

5. An electro-optical character display system, compris ing at least one electro-optically active crystal disposed between crossed polarizers and traversed by a beam of radiation, an electrode disposed on one surface of said crystal and a counter electrode disposed on the other surface of said crystal, at least one of said electrodes being in the form of a line segment representative of at least due to birefringence in the controllable crystals when aportion of a character, means for selectively connecting in their uncontrolled condition in the presence of disaid electrode and said counter electrode across a source Vergent rays i cancelled,

of potential whereby a selected portion of said beam is influenced during passage through said crystal, and means References Cited in the file of this patent for receiving said selected portion of said bearn, further UNITED S'EATES PATENTS compnslng at least one birefringent compensation crystal provided in a divergent path of radiation, the thickness ,142,106 Boswau Jan. 3, 1939 and refractive index difference of said compensation crys- 2,780,958 Wiley Feb. 12, 1957 tals being selected so that the phase difference occurring 1 2,928,075 Anderson Mar. 8, 1960 

